This invention relates generally to power capacitor assemblies used in power transmission and distribution systems and, more particularly, to an inductance element for a power capacitor assembly.
Electrical utility networks supply power for both commercial and residential purposes. In typical power networks, transmission lines carry both real and reactive power. The reactive power usually energizes cables, switches large industrial gears, and energizes reactive loads (e.g. motors and transformer coils), whereas the real power carried by the distribution lines performs the work at the load connected to the line. Multiphase, alternating current (AC) electrical networks tend to undergo undesirable reactive power changes when subjected to the repeated connection and disconnection of large reactive loads onto and off of the distribution line. The reactive power changes that occur in the network generally result in low system efficiency and high-energy losses. More specifically, these energy losses occur, for example, when large inductive loads are connected to the distribution lines, thus producing in inordinate amount of lagging reactive current in the line. Accordingly, it is desirable to provide a compensation system to correct for such reactive power in the electrical line.
For this purpose, one compensation technique is to use a number of high voltage capacitor banks, each of which can be selectively connected or disconnected from the network depending on the particular need for correction. High voltage capacitor units typically have both very low equivalent series resistance and inductance values, with the inductance values typically being on the order of 1 microhenry (xcexcH). A low series resistance is desirable in order to limit power losses, while a low inductance is sometimes desirable in applications where it is advantageous to transfer energy out of a capacitor very rapidly. However, such low inductance characteristics can result in high inrush current transients when the capacitor is energized, particularly when there are multiple capacitor banks at the same location. In turn, high inrush current transients impose severe duty on associated switching devices, fuses, and the capacitors themselves.
In order to control high current transients, it is known in the art to externally connect a bank of reactors, or coils, to power capacitor assemblies. These external reactors act a xe2x80x9ccurrent chokexe2x80x9d which protect the capacitors and help to maintain rupture tolerance. However, the use of large external components, such as reactor banks, can contribute to increased costs and maintenance of the power network.
Integrated inductor-capacitor devices (also referred to as cap-reactors) are also known in the art. These devices may include a pair of conducting foil sheets separated by a thin layer of dielectric material. The foil sheets and dielectric material are then typically rolled together in a cylindrical fashion to form a capacitor, having connective terminals protruding from the cylinder. In order to provide this type of capacitor with an inductive function, one connective terminal is usually located at the xe2x80x9cstart endxe2x80x9d of the length of the first conductive foil and another connective terminal at the xe2x80x9cfinish endxe2x80x9d of the second conductive foil. Although this arrangement produces an increased inductance value for the device (on the order of 100-200 xcexcH), it also results in a relatively high equivalent series resistance. The series resistance can be approximately 30 times that of the entire capacitor since the length of the conducting path may be 100 or more feet long, depending on the size of the capacitor. This high equivalent series resistance would result in high thermal losses in the capacitor and adversely impact the efficiency of the power system.
A need, therefore, exists for a power capacitor assembly that addresses the aforementioned drawbacks.
The above discussed and other drawbacks and deficiencies are overcome or alleviated by an inductive element integrally configurable within a power capacitor assembly. In an exemplary embodiment of the invention, the inductive element includes a plurality of electrically conductive sheets electrically connected in parallel. The sheets are rolled and formed into a generally cylindrical body. A first electrode is in electrical contact with one end of the electrically conductive sheets, and a second electrode in electrical contact with the opposite end of the electrically conductive sheets.
In one embodiment, the plurality of electrically conductive sheets is comprised of aluminum foil. Further, a layer of dielectric film material is disposed on one side of the plurality of electrically conductive sheets. The dielectric film material is preferably comprised of polypropylene, and the first and second electrodes are comprised of tinned copper straps.